An activated sludge treatment of an aqueous organic waste is performed usually by admixing an activated sludge with the waste to be treated, aerating the mixed liquor in an aeration tank to subject the organic matter in the waste to biodegradation by microorganisms in the activated sludge and separating the sludge from the aerated mixed liquor in a settling tank, whereupon the supernatant is discharged out of the treatment system as a treated water, while the separated sludge is partly returned to the aeration tank as a return sludge and the remainder is exhausted out of the system as excess sludge. In such an activated sludge treatment of an organic matter-containing aqueous waste, the organic matter is subjected to biodegradation by the activated sludge in which certain bacteria such as Zoogloea etc., have come to prevail over others. However, in some cases, the nature of the raw waste and the conditions of the sludge treatment may rather facilitate multiplication of filamentous microorganisms, for example, filamentous bacteria, such as Sphaerotilus etc., and actinomycetes of the genus Nocardia, whereby the bulking or scumming of the sludge is caused. A classification of filamentous microorganisms found in an activated sludge is proposed by D. H. Eikelboom ("Water Research" Vol. 9, pp 365-388, Pergamon Press 1975). In the "bulked" sludge, there is often observed a filamentous bacterium which is named "Type 021N" by D. H. Eikelboom.
Bulking is a phenomenon in which the sludge becomes light and is broken up due to the multiplication of filamentous microorganisms, whereby the sludge loses its settling ability. When bulking occurs, the separation of the sludge in the settling tank becomes difficult and a part of the sludge will be discharged out of the system with the treated water, so that a sufficient amount of the return sludge is not reserved and the biotreatment performance is decreased.
When actinomycetes of the genus Nocardia multiplicates, there may occur, in addition to the possible occurrence of the bulking phenomenon, scumming in which a part of the sludge floats up to form a scum accompanied by foams which are formed by mycolic acid, an ultrahigher fatty acid, whereby the settling ability of the sludge is lost.
Hitherto, a method has been used for preventing the multiplication of the filamentous microorganisms with various biocides, in order to prevent occurrence of the bulking caused by filamentous microorganisms, wherein a quaternary ammonium salt was employed as the biocide. In Japanese Patent Application Kokai No. 112902/1984, it is disclosed that benzalkonium chloride, benzethonium chloride and mixtures of them can sterilize only the filamentous microorganisms without having influence on the other bacteria and can prevent occurrence of the bulking.
However, a quaternary ammonium salt acts in general as a bacteriocide and suppresses multiplication of bacteria other than filamentous bacteria and has an undesirable influence also on humans and animals, so that it is generally undesirable to use it. (See, for example, Japanese Patent Application Kokai No. 35895/1986).
Use of chlorhexidine hydrochloride and the like will also encounter a difficulty in controlling an effective but safe dose, since they are a powerful bactericide and, when using in an excessive amount, sterilization of other useful bacteria may be brought about. When using in an insufficient amount, the bactericidal action to the filamentous bacteria becomes insufficient. Therefore, the permissible dose range of practical use of such a bactericide is quite limited and, in addition, the effective dose of such a bactericide is different for each specific condition of the operation of the aeration tank and for each specific filamentous bacterium, so that setting of the operation condition for attaining a predetermined performance is very complex. Using a bactericide for preventing the phenomenon of bulking of the sludge, in general, the contemplated effect for attaining sterilization of the filamentous bacteria by the addition of such a bactericide appears only after a certain period of time from the addition of the bactericide, so that there is a problem in its immediate effect.
A method has been proposed for preventing the phenomenon of bulking of the sludge by adding a biostatic bactericide together with a cationic organic flocculant to the activated sludge treatment system (Japanese Patent Application Kokai No. 71286/1994).
Such a technique implies the same problems as above, so long as the biostatic bactericide constitutes the principal effective substance. A cationic organic flocculant itself has been employed for preventing the problem of the bulking (Japanese Patent Application Kokai No. 3468/1974). Here, the bulking is prevented by coagulation of the sludge by the addition of the flocculant and, hence, the contemplated preventive effect is brought about immediately after the addition of the flocculant. Here however, there is a problem in the durability of such a preventive effect and, when such a forced flocculation is successively incorporated, the preventive effect becomes faded. Combination of the biostatic bactericide with a cationic flocculant does not lead to any unexpected effect.